Magnetic and optical disks require precision surfaces with extremely low defect rates to function properly. A typical magnetic disk comprises a substrate on which multiple layers of various materials are deposited. For example, an aluminum substrate might be coated electrolessly with NiP then sputtered with thin films of Cr as an undercoat, a cobalt alloy magnetic layer and a hydrogenated carbon overcoat. Depending on the stage of the process these surfaces are not necessarily uniform. For example, after the NiP has been applied a small circular band on the surface of the disk may be textured using a laser to form microscopic bumps. This textured region is intended to provide a low stiction area for the sliders to rest during nonoperating periods. In addition to intentional variations there may be various types of defects. As the disks progress through the manufacturing process various tests and inspections are used to detect defective disks so that they may either be reworked or discarded. In addition to visual inspections, a disk may be subjected to glide tests which are sensitive to the flatness of the planar surfaces, as well as magnetic read/write tests. Due to high capacities of magnetic disks it is typically not practical to magnetically test each bit which can be stored on the disk.
Laser surface inspection of the disks if sufficiently precise may actually be superior to current magnetic tests in detecting defects. Magnetic defects are usually associated with visible defects, but the visible defects can be detected more efficiently through laser inspection even though the laser spot size is considerably larger than the area in which a bit can be recorded. Thus, laser inspection allows greater test coverage of the disk in a cost effective manner.
Various laser inspection devices are known in the art. Commonly assigned U.S. Pat. No. 5,220,617 by Bird, et al. describes a laser scanner for green sheets to detect via errors. The sheets are moved on an air track to a transport table which translates them as the scan occurs. Only one side is scanned. The system uses a rotating polygon mirror to scan and to capture the reflected light. The bright field reflected light is captured at the hole-in-plate splitter and directed to a single fibre. This channel detects contrast between the conductive paste and the green sheet. The dark field light reflected light is captured by fibers located near the surface of the object. The incident light is perpendicular, but there is a suggestion that other angles are possible. There is a start of scan mirror adjacent to the object, but its function is said to be to provide an initialization or start/stop point. The reference signal is obtained from the initial part of the green sheet. The lens assembly is a flat field telecentric anamorphic f-theta lens system. The f-theta condition corrects for the pincushion distortion. A focusing telescope converges the image down to a slit. The shaping lens system results in a collimated bundle 10.8 mm by 130 microns on the polygon. This shape is said to be selected for pickup by the fiber bundles.